1. Field of the Invention
The present invention relates in general to an engine intake system and more particularly to throttle valve control means. More specifically, the present invention pertains to engine throttle valve position control means having step motor means for moving the engine throttle valve to a desired position.
2. Description of Prior Art
Conventionally, an internal combustion engine has a throttle valve provided in an air intake passage to control the amount of intake air. For controlling the position or opening of the throttle valve in a precise manner, proposals have been made to use a step motor to operate the throttle valve. This kind of device is disclosed in, for example, Japanese Patent Public Disclosure Nos. 57-28838 and 57-116140. The control device disclosed in Japanese Patent Public Disclosure No. 57-116140 includes an accelerator position detector which applies a signal to a control curcuit for controlling the step motor connected to the throttle valve according to the detected accelerator position.
In controlling the throttle valve using the step motor, however, there is a possibility of control of the step motor being lost due to an inertial force which appears on the throttle valve body during rotation thereof. More specifically, the step motor is driven to change its rotational direction when the accelerator pedal is depressed from a released position or released from a depressed position. When such a change of rotational direction occurs, the inertial force on the throttle valve tends to keep the throttle valve rotating in the same direction as it was, which force is strong where the step motor is driven at high pulse rates.
If the inertial force becomes greater than the driving torque by the step motor in the opposite direction, the step motor will fail to drive the throttle valve to the position corresponding to the position of the pedal possibly producing an uncontrolled condition. Under this conditions, the number of steps counted in the control circuit will not be equal to the actual steps of the step motor operation so that accurate control of the throttle valve position cannot be expected. Further, once the step motor falls into an uncontrolled condition, the torque of the motor will abruptly fall and in turn it will take much time to restore a normal motor operation, whereby excess delay of control will occur.
To avoid the above problem, the step motor may be driven at slower rates. However, this creates another problem of a dull response in throttle valve control, especially when under an acceleration wherein the step motor must be quickly driven to open the throttle valve in order to increase the engine speed from a steady operation.